The invention relates to a method and apparatus for measuring co-channel interference in a mobile communications system.
Mobile communications systems, such as cellular or personal communications services (PCS) systems, are made up of a plurality of cells. Each cell provides a radio communications center in which a mobile unit establishes a call with another mobile unit or a wireline unit connected to a public switched telephone network (PSTN). Each cell includes a radio base station, with each base station connected to a mobile switching center that controls processing of calls between or among mobile units or mobile units and PSTN units.
Various wireless protocols exist for defining communications in a mobile network. One such protocol is a time-division multiple access (TDMA) protocol, such as the TIA/EIA-136 standard provided by the Telecommunications Industry Association (TIA). With TIA/EIA-136 TDMA, each channel carries a frame that is divided into six time slots to support multiple (3 or 6) mobile units per channel. Other TDMA-based systems include Global System for Mobile (GSM) communications systems, which use a TDMA frame divided into eight time slots (or burst periods).
Traditional speech-oriented wireless systems, such as the TIA/EIA-136 and GSM TDMA systems, utilize circuit-switched connection paths in which a line is occupied for the duration of the connection between a mobile unit and the mobile switching center. Such a connection is optimum for communications that are relatively continuous, such as speech. However, data networks such as local area networks (LANs), wide area networks (WANs), and the Internet use packet-switched connections, in which communication between nodes on a communications link is by data packets. Each node occupies the communications link only for as long as the node needs to send or receive data packets. With the rapid increase in the number of cellular subscribers in conjunction with the rising popularity of communications over data networks such as intranets or the Internet, a packet-switched wireless data connection that provides convenient and efficient access to data networks, electronic mail, databases, and other types of data has become increasingly desirable.
Several packet-switched wireless connection protocols have been proposed to provide more efficient connections between a mobile unit and a data network. One such protocol is the General Packet Radio Service (GPRS) protocol, which complements existing GSM systems. Another technology that builds upon GPRS that has been proposed is the Enhanced Data Rate for Global Evolution (EDGE) technology, which offers even higher data rates. The enhancement of GPRS by EDGE is referred to as Enhanced GPRS (EGPRS).
In an EGPRS (Enhanced General Packet Radio Service) mobile communications system, measurements are made in traffic channels to determine co-channel interference. Traffic may be carried by a number of carriers using a predetermined frequency reuse scheme. Due to multiple uses of the same carrier frequency, co-channel interference may occur in which information (e.g., traffic) carried on one carrier (a first modulated carrier) in a first cell interferes with another carrier (a second modulated carrier) of the same frequency in a second cell. Downlink (base station to mobile unit) co-channel interference measurements are made by a mobile unit on the same RF carrier frequency as its assigned PDCH (packet data channel) during the logical idle frames (also referred to as search frames) when the serving cell is not transmitting. Since the serving base station of the cell in which a mobile unit is located is not transmitting traffic during these idle frames, the signals received by the mobile unit are due in large part to interference (including co-channel interference) from other cells and noise. Interference measurements are possible in an EGPRS system as well as other conventional cellular systems because neighboring base stations may be communicating during an idle frame of a serving base station.
However, in systems in which base stations are time synchronized, such as EGPRS Compact systems, idle frames in neighboring cells assigned the same carrier frequency occur generally at about the same time. EGPRS Compact provides for deployment of packet data services in a reduced spectrum (e.g., less than 1 MHz). In systems with time synchronized base stations, the idle frames substantially coincide on all RF carriers in all cells. Thus, a mobile unit that is in an idle period in one cell (the serving cell) may be unable to measure signaling from another cell (also idle) due to the time synchronization because there is theoretically nothing to measure during an idle period. Therefore, a need exists for a technique to perform co-channel interference measurements in systems in which base stations and cells are time synchronized, such as in EGPRS Compact systems.